Allow configurable algorithms in verification

[tetsuo-cpp]

Summary

Support configurable signing algorithms in sigstore-go's verification flow. We already have the signing key/certificate so we can use the artifact digest algorithm information to figure out what hash function is being used.

Release Note

Documentation

[tetsuo-cpp]

CC: @ret2libc

[tetsuo-cpp]

I think it's best to get this in first and follow up with the change to restrict algorithms via a --allowed-signing-algorithms flag since it's not straightforward with the current CLI flags library.

[haydentherapper]

Now implemented.